I'm about to embark on GMing a TL2 fantasy campaign and approaching this problem what I will try to do is:

-use multiple skins over a fundamental magic engine known only to me in order to stovepipe mage types such that while they may have some mechanistic understanding of their own magic use, the capabilities and methods of other mages will be obscure to them;

-base threshold levels on CP levels;

-use a high threshold but make recovery slow;

-base spell threshold costs on the scale/scope of the effect, and make the scale and scope of effect partially unpredictable;

-make spell failure complications common and relatively manageable, but subject to potentially cascading if bad luck and foolish risks combine.